Objective 16.3 of the PHS report "Healthy People 2000" is to reduce breast cancer deaths. Substantial epidemiological data suggest that pregnancy is associated with reduced breast cancer risk. Laboratory data indicate that there is a molecule present uniquely during pregnancy which can prevent the growth of breast cancer: ligand-transformed alpha- fetoprotein(AFP) stops the growth of breast cancer cells, both in vivo and in vitro. The long-term objective is to elucidate the mechanism by which AFP arrests the growth of breast cancer cells. The applicant has expressed the C-terminal domain of AFP and has demonstrated its biological activity. Therefore, the hypothesis of this predoctoral proposal is that not all of Domain III of human AFP is required for expression of its anti-oncotic activity. There are two Specific Aims: 1) To produce a truncated form of AFP(Domain III). The objectives include: evaluating the biological activity in anti-oncotic assays, evaluating the requirement for ligand (estrogen), and characterizing the biophysical properties of Domain III of AFT. 2.) To define the minimal- length analog which possess anti-oncotic activity. The research design relies heavily on recombinant DNA technology and adaptable protein expression systems. The specific methods are: to produce desired analogs by engineering cDNA templates, to express proteins in a baculovirus system, and to characterize the biological and biophysical properties of the products. To test for anti-oncotic activity, will be used: i.) inhibition of estrogen-stimulated uterine growth of the immature mouse, ii.) inhibition of estrogen-stimulated foci formation of MCF-7 cells in culture and iii.) inhibition of growth of estrogen-dependent MCF-7 tumor xenografts in mice.